1. Field of the Invention
The field of art to which this improvement pertains includes fluid operated control systems, more particularly, a control system for a vehicle having a hydrostatic transmission.
2. Description of Prior Art
Briefly, this invention constitutes an improvement in a known hydrostatic transmission control system wherein a main propulsion system, driven by a prime mover, is connected to one or more fluid traction motors which in turn drive the vehicle wheels. A fixed displacement charge pump, driven at prime mover speed, is used as a source of both fluid under high pressure and fluid under a lower pressure, with these fluid pressures in turn being utilized for the actuation of a spring-centered double-acting hydraulic motor or cylinder whose piston rod is connected to one end of the main propulsion pump swash plate. The movement of the piston rod, in one direction or the other, causes a corresponding movement of the swash plate to thereby change the displacement of the main propulsion pump. Thus, at any given time, a differential pressure is produced by the engine driven charge pump, with the magnitude of this differential pressure varying directly with the magnitude of the engine speed. The resulting pressure produces a reaction in the hydraulic cylinder that controls the swash plate which, in turn, causes a propulsion action on the vehicle drive wheels. This known prior art control system also includes a directional control valve and a differential pressure regulator valve which can influence the displacement of the propulsion pump swash plate. The high and low fluid pressures that indirectly act on the swash plate can be overridden by the differential pressure regulator valve via a control input which originates in a brake master cylinder upon the depression of the brake pedal by the vehicle operator.
Various hydrostatic transmissions and control systems therefor have been disclosed previously as exemplified by U.S. Pat. No. 3,054,263, to Budzich et al, issued Sept. 18, 1962 and U.S. Pat. No. Re. 28,113, to Howard et al, issued Aug. 13, 1974.
While the prior art hydrostatic transmission control systems will function satisfactorily in most instances, it should be recognized that manufacturing tolerances do exist and that they can and do influence vehicle performance. For example, the lower of the differential pressures used in the control system is achieved via a fixed orifice, which orifice is generally located within the charge pump which in turn is usually procured from an independent manufacturer and therefore there is only limited control over the accuracy of this fixed orifice. Furthermore, due to unavoidable manufacturing variations, there will be individual variances in the swash plate tilting moments of the main propulsion pump. In addition, there will be manufacturing tolerance variations not only in the directional control and regulator valves but also in the swash plate actuating cylinder and all of these variances or variations can and do affect vehicle performance.